This invention relates to a fluid displacement apparatus of the scroll type, such as a compressor.
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 discloses a scroll type fluid displacement apparatus including two scroll members, each having a circular end plate and a spiroidal or involute spiral element. These scroll members are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between the spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contacts along the spiral curved surfaces and, therefore, the fluid pockets change in volume. The volume of the fluid pockets increases or decreases depending on the direction of the orbiting motion. Therefore, the scroll type fluid displacement apparatus is applicable to compress, expand or pump fluids.
The principle of operation of a typical scroll type compressor will be described with reference to FIGS. 1a-1d and FIG. 2. FIGS. 1a-1d schematically illustrate the relative movement of interfitting spiral elements to compress the fluid. FIG. 2 diagrammatically illustrates the compression cycle in each of the fluid pockets. FIGS. 1a-1d may be considered to be end views of a compressor wherein the end plates are removed and only the spiral elements are shown.
Two spiral elements 1 and 2 are angularly and radially offset and interfit with one another. As shown in FIG. 1a, the orbiting spiral element 1 and fixed spiral element 2 make four line contacts as shown at four points A, B, C, D. A pair of fluid pockets 3a and 3b are defined between line contacts D-C and line contacts A-B, as shown by the dotted regions. The fluid pockets 3a and 3b are defined not only by the wall of spiral elements 1 and 2 but also by the end plates from which these spiral elements extend. When orbiting spiral element 1 is moved in relation to fixed spiral element 2 by, for example, a crank mechanism, so that the center O' of orbiting spiral element 1 revolves around the center O of fixed spiral element 2 with a radius of O--O', while rotation of the orbiting spiral element is prevented, the pair of fluid pockets 3a and 3b shift angularly and radially towards the center of the interfitted spiral elements with the volume of each fluid pocket 3 a and 3b being gradually reduced, as shown in FIGS. 1a-1d. Therefore, the fluid in each pocket is compressed.
Now, the pair of fluid pockets 3a and 3b are connected to one another while passing the stage from FIG. 1c to FIG. 1d and as shown in FIG. 1a, both pockets 3a and 3b merge at the center portion 5 and are completely connected to one another to form a single pocket. The volume of the connected single pocket is further reduced by further revolution of 90.degree. as shown in FIGS. 1b, 1c and 1d. During the course of revolution, outer spaces which open in the state shown in FIG. 1b change as shown in FIGS. 1c, 1d and 1a, to form new sealed off fluid pockets in which fluid is newly enclosed.
Accordingly, if circular end plates are disposed on, and sealed to, the axial facing ends of spiral elements 1 and 2, respectively, and if one of the end plates is provided with a discharge port 4 at the center thereof as shown in figures, fluid is taken into the fluid pockets at the radial outer portion and is discharged from discharge port 4 after compression.
Referring to FIG. 2 and FIG. 1, the compression cycle of fluid in one fluid pocket will be described. FIG. 2 shows the relationship of fluid pressure in the fluid pocket to crank angle, and shows that one compression cycle is completed at a crank angle of 4.pi., in this case.
The compression cycle begins (FIG. 1a) when the fluid pockets are sealed, i.e., with the outer end of each spiral element in contact with the opposite spiral element, the suction phase having finished. The state of fluid pressure in a fluid pocket is shown at point h in FIG. 2. The volume of the fluid pocket is reduced and fluid is compressed by the revolution of the orbiting scroll until the crank angle reaches 2.pi., which state is shown by the point l in FIG. 2. Immediately after passing this state, and hence, passing point l, the pair of fluid pockets are connected to one another and simultaneously are connected to the space filled with high pressure, which is left undischarged at the center of both spiral elements. At this time, if the compressor is not provided with a discharge valve, the fluid pressure in the connected fluid pockets suddenly rises to equal the pressure in the discharge chamber. If, however, the compressor is provided with a discharge valve, such as a reed valve, the fluid pressure in the connected fluid pockets rises slightly due to the mixing of the high pressure fluid and the fluid in the connecting fluid pockets. This state is shown at point m in FIG. 2. The fluid in the high pressure space is further compressed by revolution of the orbiting scroll until it reaches the discharge pressure. This state is shown at point n in FIG. 2. When the fluid in the high pressure space reaches the discharge pressure (as determined by the spring constant of the reed valve and the area of the discharge port), the fluid is discharged to the discharge chamber through the discharge port by the automatic operation of the reed valve. Therefore, the fluid in the high pressure space is maintained at the discharge pressure until a crank angle of 4.pi. (point o in FIG. 2) is reached.
Accordingly, one cycle of compression is completed at a crank angle of 4.pi., but the next cycle begins at the mid-point of compression of the first cycle as shown by points h', l' and m' and the dot-dash line in FIG. 2. Therefore, fluid compression proceeds continuously by the operation of these cycles.
There are advantages to designing a scroll type compressor wherein each compression cycle is completed at a crank angle of 6.pi., rather than 4.pi.. Such a compressor naturally would have a greater number of turns in its spirals. FIG. 3 illustrates the compression cycle of fluid in this compressor.
Referring to FIG. 3, the pressure changes in one fluid pocket due to the orbital motion is shown by points h, l, m, n, o, and p. In comparison with the above mentioned compressor cycle which is completed at a crank angle of 4.pi., the pressure differential between the adjacent fluid pockets of this compressor will be smaller. Therefore, the amount of fluid leakage from the higher pressure fluid pockets to the lower pressure pockets across the line contacts between the sprial curved surfaces is reduced to thereby improve the volumetric efficiency. Furthermore, with the greater number of turns of the spiral elements the swept volume of the compressor advantageously is made larger.
There are disadvantages to this configuration, however. The axial length or height of the spiral elements of a conventional scroll type compressor is uniform so that, with a greater number of turns of the spiral elements, the internal compression ratio of the compressor is increased, thereby increasing the power consumption of the compressor. If this compressor is used in applications requiring a lower compression ratio, overcompression results, the compression cycle in this instance illustrated in FIG. 3 by points h, l, n", o and p. This cycle resembles that for a compressor which is not provided with a reed valve--a cycle indicative of excessive power loss.